[SCzEngrgGroup]

I've been doing a fair amount of work lately that involves milling on thing stock. I've been doing the work-holding using screws - first around the perimeter, then, after drilling some holes, more in the middle of the parts. A vacuum chuck seems a much neater solution, but I have no idea how well they can really hold things, and, more importantly, how good/expensive a pump I'd need.

My approach would be to take a piece of perhaps 1" stock, hog out the bottom as a plenum, drill the top with a matrix of holes, cover the plenum with a sealed plate, drill through the side walls for one or more vacuum fittings. Turn the whole thing over, surface it, cut a groove around the outside for a seal, then make multiple aluminum or plastic top fixture plates with holes selectively drilled to let the vacuum in under the part(s), grooves for seals to be made using window screen spline (which I happen to already have 6000 feet of....).

Regards,
Ray L.

[hoss2006]

Just faved this yesterday to go on the todo list, seems plenty strong.

[ame="http://www.youtube.com/watch?v=-tNc0zom0xM&feature=channel_video_title"]VACMAGIC VM300 - YouTube[/ame]

[TXFred]

I made one at my last job. We had to do a lot of engraving of gravograph plastic, and this thing was a lifesaver. I made it long enough that I could take a whole sheet of the plastic, shear it into strips, and fit it on the table.

I learned enough from this one that the next one I make will be better. Here's my wisdom.

1. Only a single vacuum port is needed unless you plan to set up multiple pieces at a time. I put the port near the bottom left corner, and made that corner the origin for all my programs. You can see that I put in lots of holes for ports. This was a mistake, because I had to divert the gasket around each one of these holes.

2. I used 1/8" closed cell foam tube from MSC as my gasket material.

3. The grooves were milled with a 1/8" end mill. Feedrate was kept low to create the smoothest possible surface.

4. Face the top after the grooves are milled. Then chamfer the edges of the grooves, or they'll tear up your seal. Round the corners of each raised square, for the same reason. Once that's all done, flip it over and face the bottom.

5. The grooves should be spaced so that the distance from the outside edge of one groove to the opposite edge of the next is a fraction of an inch. Your stock will likely be measured in whole inches. You need to be sure that the stock completely overlaps the seal. If your grooves are on 1/4", 1/2" or 1" centers, this won't happen.

6. The grooves should be about .08" deep or less. The foam compresses a lot, and you want a good seal. I made my grooves 0.1" deep, and sometimes had a hard time getting a good seal. Put a vacuum gauge on your pump so you can verify the seal. Also put a ball valve on the pump, so you can quickly apply or release vacuum without power cycling the pump.

7. The Harbor Freight vacuum pump works fine, but does put out some oily smoke. Also, it is not compatible with flood coolant. It will inhale any coolant that leaks past the seal, and that coolant winds up in the pump's crankcase.

8. When programming, touch off your Z to the table, then jog up to set your zero. Since your zero is relative to the table, you can do clever tricks like cutting almost all the way through material without breaking the seal or milling into the table. And when you're done you can break the material into pieces along the lines that you milled.

[Bubba]

Ray,
I made one to use for making pcbs (haven't done it yet) but my surface was grooved on 1" centers .065" deep. At the intersection of several of them, I drilled and tapped for 6-32 button head screws. I used O ring cord to fill the perimeter of what I needed for any sized job and used the button heads to close any unneeded ports. I use a vacuum pump that was originally for home health care oxygen concentrator.
The last time I used it, was to hold a piece of 1/4" Al plate so I could engrave it mostly with a 1/8" end mill. Didn't move the whole time!

OH, have a filter on the inlet to the vac pump to catch any drips that made it though as I used flood coolant the whole time I was engraving.

[SCzEngrgGroup]

Fred/Art,

A wealth of good information there. Thanks!

That H-F pump is the very one I already have, from repairing the A/C on my BMW a few years back, so that's good news. I think the solution to the coolant problem would be to put a condenser and liquid trap (a small reservoir with the inlet and outlet at the top, and room for fluid to collect in the bottom) in the vacuum line, no?

I would be using this almost entirely for aluminum sheet - mostly 1/8" thick. I normally machine it with tools, sometimes up to 1/2" diameter, but generally DOC with large tools would be small-ish, perhaps 1/16". Would that present any problems in holding?

I would also expect to make a custom top-plate to fixture each part. Rather than having the square pattern of channels, these plates would have vacuum pockets cut in them specific to the exact parts being cut, and would mount on top of the plate with the square pattern. Does that make sense?

Regards,
Ray L.

[Bubba]

Ray,
This is basically what I used on the inlet to the vac. pump
and I also put a smaller one on the exhaust to quieten it down a lot.

The holding force is going to based on the area (sq in) being held down and the pressure applied. I have read that "small" parts may not be held well enough and may need some "fences" to provide some assistance.

"I would also expect to make a custom top-plate to fixture each part. Rather than having the square pattern of channels, these plates would have vacuum pockets cut in them specific to the exact parts being cut, and would mount on top of the plate with the square pattern. Does that make sense?"

Yep and I think it will work! Best way to find out is to try it.

[TXFred]

That H-F pump is the very one I already have, from repairing the A/C on my BMW a few years back, so that's good news. I think the solution to the coolant problem would be to put a condenser and liquid trap (a small reservoir with the inlet and outlet at the top, and room for fluid to collect in the bottom) in the vacuum line, no?

It depends on how much coolant is drawn in, and how long your program runs. You won't have any way to drain the trap while you're running the vacuum pump, and if it fills up you're back to square one.

In my setup, with the less than ideal seal, the pump's sump filled with coolant in about a minute.

I would be using this almost entirely for aluminum sheet - mostly 1/8" thick.

I used that vac table for plastics, so I don't have any good data on metals.

For comparison, I've machined 1/16" brass sheet using nothing but double sided scotch tape to hold it to an MDF table. The vacuum table exerts much more force than the tape does. However, I never tried a 1/2" end mill. I stuck to 1/4" and smaller. My concern is that the cutting force will lift the work over a small area, leading to break through and/or vacuum loss.

If you confine the vacuum to the area of the finished part, you lose holding force. But you also make it possible to cut the outer profile all the way through. It's a trade off.

Just wear good safety gear during your test cuts, in case you do lose vacuum.

Frederic

[SCzEngrgGroup]

It depends on how much coolant is drawn in, and how long your program runs. You won't have any way to drain the trap while you're running the vacuum pump, and if it fills up you're back to square one.

I'll just use a garbage can! That way it'll drain the 15 gallon coolant reservoir before the garbage can fills up, and protect the vacuum pump! :-)

I've used double-sided tape with reasonably good results. If a vacuum setup can do better than that, it should meet my needs. Thanks!

Regards,
Ray L.

[Geof]

I'll just use a garbage can!....Ray L.

If you subject a garbage can to meaningfull vacuum you will end up with an imploded garbage can.

[rwskinner]

I often use my vacuum pump to bleed brake systems, suck out oil and other fluids. I rigged up a large glass jar and drilled two holes in the top and pulled my rubber surgical tubing thru there.

All the fluids collect in the jar and I have not had any contamination issues yet. The jar is made for a vaccum because they seal foods in them, and the glass one allows you to see the level. Of course I've been lucky and hadn't broke my glass jar yet but being in a shop it's bound to happen.


Richard

[mhackney]

I've been interested in vacuum hold down systems for a while. But I am not sure how they would perform with my parts. Here is a typical sheet I cut:



This is .065" aluminum or thinner brass or nickel silver 4" by 9". Currently, I hold down around the edges (on my tool plate), mill the holes/ports in the 3 disks. Then mill the perimeter with 3 tabs on each disk. They are a PIA to deal with and it would save me a lot of time if I could just do a clean cut around the perimeter to separate the parts. I have been working in a dedicated tooling plate for this sheet that has a tapped hole located at the centers of each disk. I would cut the stock to dimension, pre-drill the center holes (on the mill) and then mount the sheet on the mill using dimensioned pins in the holes in the tooling plate to align the stock. Then I'd mill the cutouts/ports as usual. Pause the program after they are cut and replace each of the alignment pins with a screw and a large "washer" to hold down the disk. These washers would be about .25" diameter smaller than the disk so they hold close to the edge (and I'd probably dish the underside so the porch is at the edge). Then I could cut the profiles cleanly.

Although this would work, it is a bit time-consuming (but maybe produce better results than dealing with tabs) since I have to pre-drill the center holes in one setup, use a different setup to mill, and relocate clamps during each run. A vacuum clamping system seems like it would be the best option but with all of the holes in my parts (there isn't much metal left!) I don't know how well they would work.

Anyone with any experience with this?

The Mitee-Bite system Hoss linked to does not use a vacuum pump, it uses air pressure. That seems like an ideal solution for both cost and to eliminate potential of contaminating a vacuum pump with coolant.

cheers,
Michael

[SCzEngrgGroup]

I'd guess you'd have trouble holding those parts with vacuum. Vacuum holding depends on having large areas subjected to the vacuum. You don't have a lot of area to work with there. By the time you put some seals in, you'll have even less.

Regards,
Ray L.

[SCzEngrgGroup]

The Mitee-Bite system Hoss linked to does not use a vacuum pump, it uses air pressure. That seems like an ideal solution for both cost and to eliminate potential of contaminating a vacuum pump with coolant.

cheers,
Michael

It has to be using a venturi to generate the vacuum from airflow. I'm surprised the airflow requirement is relatively low (IIRC, 4CFM@70PSI). I would've expected considerably higher. But, still, that's enough to keep most home compressors running very often, if not continuously. I think you'd still need at least about a *real* 5HP compressor. And you'll have continuous noise as well, since that 4CFM is getting dumped into your shop.

Regards,
Ray L.

[TXFred]

I'd guess you'd have trouble holding those parts with vacuum. Vacuum holding depends on having large areas subjected to the vacuum. You don't have a lot of area to work with there. By the time you put some seals in, you'll have even less.

This is where the method I described earlier comes in handy. Cut almost all the way through the part. Once it's done you can easily punch out the finished parts from the metal sheet. Since the cutter never breaks through, there's no vacuum loss. So you can apply vacuum to the entire part, holes included.

If using this method, I would suggest using an engraving tool to make the cuts. An engraver has a single vertical flute, so it doesn't try to lift the work off of the vac table. I've tried it with an end mill, and cutting forces can lift the work in a local area, which causes a break through and vacuum loss.

Frederic

[wizard]

The biggest problem is that you need area to develop clamping force. The more the better. In the best of circumstances you seldom even get close to 14lbs per square inch.

The use of a rubber seal isn't always required, well matted parts will hold without the rubber seal. However well mated isn't a given. The rubber seal provides an additional advantage in preventing lateral sliding of the work piece.

At times you simply don't have the area to hold effectively against lateral forces. What you can do then is to provide a lip or fence on your chuck to help the part resist sliding across the face of the chuck.

Particle board can be used as the chucks face. Obviously this needs to be a dry process but particle board is porus enough that you can pull a vacuum through it. You may be able to get buy with some cut throughs using particle board as the material will meter the vacuum to some extent. If you have a set of parts to produce you can use a sealer to stop vacuum lost in the cut out areas on your part.

Vacuum chucks can be very easily be made to order. This means you can tailor the design of the vacuum ports to your part. Further a vacuum chucks can allow you to get started on a part and then add mechanical clamping after deleted material exposes the underlying chuck.

[Team Pierson]

Great discussion. A lot of great tips out there. I'll put in my 2 cents from a manufacturer's perspective.

First, take a look at our vacuum chuck system. You'll notice that we use a chuck that requires gasketing. This is definitely the most efficient form of chucking since the maximum vacuum is always concentrated within the gasketed area. For those systems where the chuck is similar to a reverse air-hockey table, the vacuum is strongest above the vacuum inlets. For light cuts, this is acceptable, especially if there's a large surface area. These types of chucks, along with the particle board variety, require a high vacuum flow and not necessarily a high vacuum level... again because of a large part surface area. Our system has a high vacuum level but with a tradeoff of a low vacuum flow. We can get away with this because we're gasketing off the part and once you reach max vacuum there is no vacuum flow.

Hope this helps.

[pete from TN]

Nice to see your response here. I have been to your website often and I really like the ideas and ingenuity you exhibit in your products. I also applaud that they are all designed and manufactured here in the USA. Honestly I cannot afford your workholding systems just yet but they do interest me. I also appreciate the fact that you make parts of them available to customers so they can possibly create their own systems. I can see the beauty of the pallet system you sell. It seems like it would be very adaptable to many different kinds of fixture setups. How does that double boss in the bottom of the fixture plate work exactly? Peace

Pete

[dak1]

To reduce the lifting force of the cutter, you can get "down cut" tools (right hand cut, left hand helix) wich would apply the cutting force downward toward the workpiece. (A left hand cut, left hand helix combo would still pull upward, just with spindle reversed). Chip evacuation may become a different problem though with a downcut tool. I don't know where to tell you to find them, but heres a link to a reletive thread.

http://www.cnczone.com/forums/genera...end_mills.html

[ezh]

TXFRED: Thank you for the great description. Can you tell us exactly which 1/8" tape you got from MSC ... there are several in the catalog.

My understanding: if I cut the groove .080" deep, then it seals well, but the tape compresses all the way back into the groove, so the workpiece ends up solidly located against the top of the plate. Is this right?

Thanks again

[Team Pierson]

As far as making your own vacuum fixture, here's a couple products that should help. Our gasket is 1/8" in diameter and we offer a .118" dia endmill so the gasket has a slight squeeze that holds it in place.

Vacuum Gasket: SmartVac II Vacuum Gasket (20')
.118" Endmill: SmartVac II "Gasket Slot" Endmill

Just machine your gasket groove .100" deep and the part will compress the gasket (it's a soft closed cell foam) into the groove and your part will have 100% contact with the fixture. Oh, and if you need a vacuum pump, ours runs off shop air and only uses 0.8 CFM so it's pretty efficient and doesn't mind coolant either. Feel free to contact us if you need specific help.

Vacuum Control Unit: SmartVac II Vacuum Control Unit